PNP: What's in a Name?

The name PNP was introduced by Eisenberg and Chen because it has important physical meaning beyond being the first letters of Poisson-Nernst-Planck. PNP also means Positive-Negative-Positive, the signs of majority current carriers in different regions of a PNP bipolar transistor. PNP transistors are two diodes in series PN + NP that rectify by changing the shape of the electric field. Transistors can function as quite different types of nonlinear devices by changing the shape of the electric field. Those realities motivated Eisenberg and Chen to introduce the name PNP in 1993.The pun “PNP = Poisson-Nernst-Planck = Positive-Negative-Positive” has physical content. It suggests that Poisson-Nernst-Planck systems like open ionic channels cannot be assumed to have constant electric fields. Indeed, the equations of electrodynamics make it more or less impossible that a channel have a constant electric field, if there is permanent charge nearby. The electric field must be studied and computed because its change of shape is unavoidable for charged channels, and the shape of the electric field is likely to be important in the function of biological systems, as it is in semiconductor systems.